Devsayanai Ekadasi (Shayana Ekadasi, Tholi Ekadasi or Maha Ekadasi or Ashadi Ekadasi) is observed on Shukla paksha Ekadashi in Ashada month. Devsayani Ekadasi 2012 date is June 30, Saturday. This is observed as Tholi Ekadashi (first Ekadasi) or Maha Ekadasi or Pedda Ekadashi (big Ekadasi) in Andhra Pradesh. It is celebrated as Jagannath Maha Ekadashi in Puri, Orissa.
Devshayani Ekadashi marks the beginning of Chaturmas puja or the vrata of four holy months begins. This Ekadasi is also known as Harivasara Ekadasi and Vishnu Sayana Ekadasi during when Lord Vishnu goes on to cosmic sleep. Dakshinayanam or Pitrayana or the second half of Hindu year begins during Maha Ekadasi.
Here you can read Devshayani Ekadasi vrat katha or the story of Devashayani Ekadashi.